Refrigeration systems with control mechanisms allow for the regulation and control of an environmental parameter such as temperature within the system. These control mechanisms rely on one or a plurality of measured values to regulate a system. A user sets a desired temperature of a fluid or gas in a volume via an interface. A control mechanism using an internal algorithm alternatively turns on or turns off one or a plurality of active elements capable of acting upon the temperature and ultimately regulates the temperature as it varies over time and under variable load conditions.
In some applications, such as cooking or scientific research, the temperature of a specimen must be controlled precisely, often to within a fraction of a degree. In the past, the precise control of the temperature has often been the sole design parameter of these systems, but in recent years, new-generation control systems must also be respectful of their environment. Problems related to different control mechanisms include creation of residual heat, high and often uninterrupted power consumption, and parasitic noise associated with cooling mechanisms such as fans. For example, office workers may work in an environment where light fixtures produce noise when energized, computers are cooled by noisy fans, air is circulated via a noisy overhead ventilator, and windows are often poorly insulated from external noise. As a consequence, noise pollution is created, which can lead to occupational concerns in the workplace. As new devices are produced and sold, the need for cost-effective, wisely controlled, energy-efficient, and low-noise products has become a reality.
Within the scope of this patent, one technology is used to describe embodiments of the invention, namely, control systems associated with refrigeration cycles. One of ordinary skill in the art will understand that noise reduction, while of great importance to the field associated with the described technology, should not be viewed as specific to refrigeration technology and could be applied with equal or greater force to other technologies. By way of example, video cameras equipped with microphones benefit from low background noise and low residual noise produced by the different components operating within the camera itself. Further, electronic equipment held by camera operators or stored in the proximity of a camera benefits from a low residual noise control system.
In one possible embodiment, a vapor-compression, closed-loop refrigeration system can be used to cool or heat a fluid. A plurality of elements within this closed-loop system can be monitored to control the temperature of the fluid. A fan blows air over a condenser coil and a compressor to cool these elements and dissipate heat. These fans, generally in the shape of small propellers, rotate around an axis and blow air from the fan blades to cooling fins. These fans are often left to cycle on and off based on the cooling needs of the system as regulated by the control mechanism.
Fans are means of forced convection. Most of the time, a fan capable of controlling the temperature of an element is not in operation when the element is at its optimum temperature. Rather, fans are energized only when the element exceeds a desired temperature or temperature range. Convective cooling often remains very effective even when fan speed is low. Because convective fans are either on or off, they are often turned on and may burn up quickly. The presence or absence of noise from changes in operating status are often bothersome. Known control mechanisms turn the fan off and wait until the system heats beyond a desired value, at which time the fan is turned on to its maximum speed. This system is not optimized to control the temperature very precisely. Another embodiment is to use a variable-speed fan, to fix gradient fan speeds based on different levels of input power, and to correlate these speeds with the compressor outlet temperature or the pressure at the compressor outlet. These changes in speed also result in abrupt changes in the system and associated noise disturbances. While an increase in speed may significantly increase the noise produced by the fan, it may not significantly improve the heat dissipation capacities. What is needed is a control mechanism capable of regulating a fan while producing little noise but without creating adverse temperature variations in the fluid to be cooled by the fan as the fan is regulated.
In addition, fan controls can be stand-alone or part of a proportional integral derivative controller (“PID controller” or “PID”). PID controllers are present in a wide range of instruments and can be used to control temperature, pressure, or other parameters. For example, cooking instruments, laboratory testers, and therapeutic equipment use fluid baths operating at a controlled temperature. Under normal operating conditions, as the temperature of a fluid is controlled, energy drained or added to the fluid by a body in contact with fluid alters the fluid temperature unless a secondary heat flux is added via a heater or a cooler to regulate the desired setpoint.
Control mechanisms must compensate in real time to introduce or remove heat from the fluid to stabilize the temperature at the setpoint. Based on a time-incremental method of measure, sensors measure the variable to be controlled on a real-time basis. Likely error and fluid variable change can be anticipated by using a model that reviews linear variation of variations and associated error over past-time increments using a proportional controller, reviews the derivative of change of the error over past-time increments using a derivative controller, and uses an integral controller to calculate the rate of change of the error over time. The PID controller includes a proportional component (P), an integral component (I), and a derivative component (D) used in concert to correct the error. Each of these three components includes a tuning factor called the “gain.”
The use of a PID controller with variable gains to minimize the error of a setpoint variable, such as temperature in a fluid bath, is known, as is the use of a first PID to control a heat source and a second PID to control a refrigeration system of a temperature control device. In yet another known device, an external control with a proportional signal based on a temperature of the refrigerant system is supplied to a single PID controller. This external signal allows for the reverse calculation of an unmeasured parameter in the system to better operate a single PID controller. What is needed is an improved system based on PID control technology capable of correcting a fluid temperature error while limiting noise associated with controlling a fan used in the system.